1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrostatic recording process or an electrophotographic process. More specifically, the present invention relates to an image forming apparatus such as a copying machine, a printer, and a fax machine.
2. Related Background Art
In the image forming apparatuses such as laser beam printers using a two-component developer mainly composed of toner and carrier, toner is consumed each time an image is formed so that the developer density (mixture ratio between toner and carrier) within a developing device varies. For this reason, in order to maintain the developer density constant, a technique for accurately detecting the developer density becomes important.
The detection of the developer density is usually performed by an optical sensor. A patch latent image is formed on an electrically charged photosensitive drum (drum-shaped electrophotographic photosensitive member) at a predetermined latent image contrast. The patch latent image is then developed using a two-component developer contained in a developing device to obtain a patch image. Light is irradiated to the patch image formed on the photosensitive drum by an optical sensor to detect the developer density based on the magnitude of reflected light obtained at this time.
An initial density, which serves as the reference for the magnitude of reflected light, is set when installing the image forming apparatus and when replacing the developer. The initial density is set to such a density (hereinafter referred to as the “reference density”) that allows the sensitivity of the optical sensor to be optimized. When installing the image forming apparatus and when replacing the developer, developer is fed from a sealed container into a developing container of the developing device. The developer within the sealed container is moisture-conditioned to the moisture content within the sealed container (hereinafter referred to as the “container moisture content”). While the developer is being fed into the developing container of the developing device, however, it comes into contact with the outside air so that it is moisture-conditioned to a moisture content (hereinafter referred to as the “ambient moisture content”) detected by an environmental sensor provided in the image forming apparatus.
In the meantime, the triboelectricity (triboelectrification charge amount) of the developer also changes from a value corresponding to the container moisture content to a value corresponding to the ambient moisture content. Accordingly, it is common that the initial density is so controlled as to become the reference density in all environments, by changing the latent image contrast in accordance with a detected value of the environmental sensor.
In recent years, in order to achieve higher image quality, toner and carrier contained in developer tend to be reduced in particle size. Such developer including toner and carrier with reduced particle sizes have a larger surface area as a whole as compared with developers having the same weight but larger particle sizes. An increased surface area means a longer time required for moisture-conditioning the developer. Accordingly, when installing an image forming apparatus and when replacing developer, the developer is not sufficiently moisture-conditioned before setting the initial density of a patch image, so that the initial density is set before the triboelectricity of the developer becomes a value corresponding to the moisture content detected by the environmental sensor.
A moisture content (the mass of water contained in 1 m3 of air) within a sealed container is dependent on the environment under which a developer is filled into the sealed container. It is generally about 10 g. Since a sealed container is hermetically sealed, the container moisture content varies little even after passage of a long period of time. For instance, consider a case where the moisture content in the sealed container is 10 g and the moisture content detected by the environmental sensor when installing the image forming apparatus and when replacing developer is 1 g. In this case, the latent image contrast is set to a value corresponding to the moisture content of 1 g, whereas the triboelectrification charge amount of the developer is determined to a value corresponding to the moisture content of 10 g, causing a phenomenon in which the density of the patch image largely deviates from the reference density.
As a result, it is impossible to perform the setting of the initial density, or even if it is possible, the initial density largely deviates from the reference density.
As in the latter case, if an initial density deviates largely from a reference density, the density detection by a density sensor is performed within a density range where the sensor sensitivity is poor, so that the control of the developer density becomes unstable. Consequently, the density of a patch image, which is formed when developer is moisture-conditioned and thus the triboelectricity thereof changes, becomes low and due to an output from the density sensor that has detected this, toner is supplied into the developer so as to increase the developer density, with the result that the developer density is raised to an abnormally high level. This results in fogging or toner scattering, or in some extreme cases, even an overflow of developer from the developing device.